\mainline{1...e5 2.Nf3 Nc6 3.d4}\showboard
From this point, \variation{3.d3 d5} is a good but far less
aggressive alternative.
\mainline{3...e5xd4 4.Bb5 a6 5.O-O}

The command \variation{} helps to analyse variations of a move. The number of the first move passed to this command must be the last move in the immediately previous \mainline command. For instance, in the example 3.d4 is the last move before analysing a variation, hence 3.d3 is the first move in \variation{}

The command \fenboard{ } uses FEN notation to set up the board (see further reading). For instance, the example above shows in the row 8 a rook, 5 empty squares, the king and 1 empty square; hence the "r5k1".